1. Field of the Invention
The present invention relates to an optical apparatus such as a television lens or a video lens, and more particularly, to a focus detector for automatic focus function, a lens apparatus including the focus detector, and an image pickup apparatus including the focus detector.
2. Description of the Related Art
Conventionally, there are various proposals as an automatic focus (AF) technology for an image pickup apparatus such as a camera or a video camera. For instance, the following automatic focus adjustment method is well known. Light beams from a subject passing through different exit pupil areas of an image pickup lens are guided to form images on a pair of line sensors, and the subject images are photoelectric-converted so as to obtain a pair of image signals. Then, a relative position displacement amount between the image signals is determined. Based on this displacement amount, a defocus amount of the subject is calculated so as to drive the image pickup lens for the automatic focus adjustment.
This AF system using phase difference detection can determine an in-focus position of a focus lens from a subject distance, and hence has a feature that focusing can be performed faster than in a contrast AF system.
Japanese Patent Application Laid-Open No. 2010-66712 discloses a method of increasing a defocus range of focus detection by decreasing the number of pixels to be subject to correlation computation on the pair of line sensors to be used for normal phase difference detection so as to increase a pixel shift amount for the correlation computation, in order to reduce the probability of automatically entering a scan AF mode for detecting a focus shift while driving the focus lens when the focus detection by the phase difference detection method cannot be performed.
However, although the method of Japanese Patent Application Laid-Open No. 2010-66712 can increase the defocus range of focus detection, detection accuracy is deteriorated because the number of pixels to be subject to correlation computation is reduced. In addition to that, when plural subject images of plural subjects at different subject distances exist on the pair of line sensors of the AF sensor respectively, there is a problem that an erroneous detection is apt to occur (subject images of different subjects existing on different line sensors are erroneously detected as subject images of the same subject).